1. Field of the Invention
This invention relates generally to magnetoresistive read sensors and particularly to the free layer formations of such sensors operating in a tunneling magnetoresistive (TMR) configuration and current-perpendicular-to-plane GMR configurations.
2. Description of the Related Art
In simplest form, the usual giant magnetoresistive (GMR) read sensor consists of two magnetic layers, formed vertically above each other in a parallel planar configuration and separated by a conducting, but non-magnetic, spacer layer. Each magnetic layer is given a unidirectional magnetic moment within its plane and the relative orientations of the two planar magnetic moments determines the electrical resistance that is experienced by a current that passes from magnetic layer to magnetic layer through the spacer layer. The physical basis for the GMR effect is the fact that the conduction electrons are spin polarized by interaction with the magnetic moments of the magnetized layers. This polarization, in turn, affects their scattering properties within the layers and, consequently, results in changes in the resistance of the layered configuration. In effect, the configuration is a variable resistor that is controlled by the angle between the magnetizations.
The magnetic tunneling junction device (TMR device) is an alternative form of GMR sensor in which the relative orientation of the magnetic moments in the upper and lower magnetized layers (also called electrodes in this configuration) controls the flow of spin-polarized electrons tunneling through a very thin dielectric layer (the tunneling barrier layer) formed between those electrodes. When injected electrons pass through the upper electrode, as in the GMR device, they are spin polarized by interaction with the magnetization direction (direction of its magnetic moment) of that electrode. The probability of such an electron then tunneling through the intervening tunneling barrier layer into the lower electrode then depends on the availability of states within the lower electrode which the tunneling electron can occupy. This number, in turn, depends on the magnetization direction of the lower electrode. The tunneling probability is thereby spin dependent and the magnitude of the current (tunneling probability times number of electrons impinging on the barrier layer) depends upon the relative orientation of the magnetizations of magnetic layers above and below the barrier layer.
In what is called a spin-filter configuration, one of the two magnetic layers in both the GMR and TMR has its magnetization fixed in direction (the pinned layer), while the other layer (the free layer) has its magnetization free to move in response to an external magnetic stimulus. If the magnetization of the free layer is allowed to move continuously, as when it is acted on by a continuously varying external magnetic field, the GMR and TMR device each effectively acts as a variable resistor and it can be used as a read-head.
The difference in operation between the GMR sensor discussed first, and the TMR sensor just now discussed, is that the resistance variations in the former are a direct result of changes in the electrical resistance (due to spin polarized electron scattering) within the three-layer configuration (magnetic layer/non-magnetic, conducting layer/magnetic layer), whereas in the TMR sensor, the amount of current is controlled by the availability of states for electrons that tunnel through the dielectric barrier layer that is formed between the free and pinned layers.
When used as a read head, (called a TMR read head, or “tunneling magnetoresistive” read head) the free layer magnetization is moved by the influence of the external magnetic fields of a recorded medium, such as is produced by a moving hard disk or tape. As the free layer magnetization varies in direction, a sense current passing between the upper and lower electrodes and tunneling through the dielectric barrier layer varies in magnitude as more or less electron states become available. Thus a varying voltage appears across the electrodes. This voltage, in turn, is interpreted by external circuitry and converted into a representation of the information stored in the medium.
A typical spin-filter GMR sensor structure is the following:
Seed/Antiferromagnetic Layer/AP2/Ru/AP1/Cu/Free Layer/Capping Layer.
A typical spin-filter TMR sensor structure is the following:
Seed/Antiferromagnetic Layer/AP2/Ru/AP1/AlOx/Free Layer/Capping Layer,
In the TMR configuration shown above (and in the GMR as well), the seed layer is an underlayer required to form subsequent high quality magnetic layers. The antiferromagnetic layer is required to pin the pinned layer, ie., to fix the direction of its magnetic moment by exchange coupling. The pinned layer itself is now most often a synthetic antiferromagnetic (SyAF) (also termed a synthetic antiparallel (SyAP)) structure with zero total magnetic moment. This structure is achieved by forming an antiferromagnetically coupled tri-layer denoted as AP2/Ru/AP1, which is to say that two ferromagnetic layers, denoted AP1 and AP2, are magnetically coupled across a Ru spacer layer in such a way that their respective magnetic moments are mutually antiparallel and substantially cancel each other. The structure and function of such SyAP structures is well known in the art and will not be discussed in further detail herein. The conducting, but non-magnetic Cu spacer layer of the GMR is replaced in the TMR by (for example) a thin insulating layer of oxidized aluminum that can be oxidized in any of several different ways to produce an effective dielectric tunneling barrier layer. The free layer in both the GMR and TMR is usually a bilayer of ferromagnetic material such as CoFe/NiFe, and the capping layer in both the GMR and TMR is typically a layer of tantalum (Ta). The bilayer choice for the free layer is necessitated by the fact that an effective free layer should be magnetically soft (of low coercivity), which is an attribute of the NiFe layer, yet it must also be an effective spin polarizer of conduction electrons, which is an attribute of the CoFe layer. We shall see below that the structure of the free layer can be significantly altered to provide an improved GMR or TMR sensor.
Superficially, the TMR structure differs from the GMR configuration by the replacement of a conducting Cu spacer layer in the GMR with an oxidized aluminum (AlOx) tunneling barrier layer in the TMR. Although this seems to be a minor substitution, the physical basis of the operation of the two structures is substantially different and, in addition, the dimensions of the various layers are also quite different.
The advantage of the TMR configuration compared to the GMR configuration is that the TMR configuration has a higher MR ratio, dR/R, (ratio of maximum resistance variation as the free layer magnetic moment changes direction, dR, to total device resistance, R), which is a measure of its efficacy as a read sensor. For example, while typical GMR ratios of GMR read sensors are less than 10%, ratios on the order of 70% have been reported for tunneling junction configurations used as MRAM devices rather than as read head sensors. The present invention will show a read head TMR sensor with a MR ratio on the order of 30%. In addition, the TMR sensor is operated in a CPP (current perpendicular to plane) mode, since it is required that the electrons tunnel through the barrier layer from the pinned layer to the free layer. GMR sensors, on the other hand, can operate either in the CPP mode or in the CIP (current in plane) mode, wherein electrons move laterally through the pinned/spacer/free layer configuration.
The CPP mode required of the TMR sensor increases overall sensor resistance, R, as the sensor layers are scaled down and made narrower and thinner to better enable their use in reading high density recorded media. To maintain a useful sensor resistance range, the thickness of the AlOx has to be reduced to less than 7 angstroms to achieve a low areal resistance, RA, in the range of approximately several ohm-μm2. As a consequence, of the decreasing RA, the MR ratio of the sensor also decreases. Thus, one of the major challenges for the design of TMR sensors is to improve the MR ratio while keeping RA low.
Much recent experimentation on GMR sensors has been directed at improvements in the free layer structure. The most common structure in both the GMR and TMR sensor had been a CoFe/NiFe bilayer, in which the NiFe layer provides the required softness, while the CoFe provides good spin polarization of conduction electrons. More recently, work has been done on improving the magnetic properties of both free and pinned layers by utilizing novel materials and laminated structures. Most notable of the novel materials has been CoFeB, an alloy of cobalt, iron and boron. Noma et al. (U.S. Pat. No. 6,493,196) teach a pinned layer formed as a tri-layer of NiFe/CoFeB/CoFe and Hosami et al. (U.S. Pat. No. 6,828,785) disclose a laminated free layer of CoFe, NiFe and CoFeB. Aoshima et al. (U.S. Pat. No. 6,046,892) show a free layer of CoFeB/NiFe and the present inventors, in Wang et al. (U.S. Pat. No. 6,844,999) teach a boron-doped (CoFeB) free layer. In fact, the use of a CoFeB free layer is taught in several patents, including Slaughter et al (U.S. Pat. No. 6,831,312), Fukuzawa et al. (U.S. Pat. No. 6,338,899) and Hayashi (U.S. Pat. No. 6,101,072). Slaughter, in particular, suggests that the amorphous nature of CoFeB is advantageous in a tunneling junction type sensor because it increases the smoothness of various layers and generally enhances the sensor's magnetic performance.
The present inventors have been investigating possible ways of improving the free layer structure for both a GMR and a TMR sensor. In the TMR sensor, the function of the free layer and the constraints placed upon the free layer are different than those in the GMR sensor and it is not necessarily true that free layer structures that are advantageously used in the GMR sensor will have similar benefits in the TMR sensor. To achieve a high MR ratio, the growth process of the barrier layer must produce a layer of great smoothness and the process by which it is oxidized must be exceptionally well controlled. In addition, the nature of the magnetic structures on either side of the barrier layer, namely the AP1 layer of the pinned layer and the contiguous portion of the free layer are also extremely important. In fact, it will be an object of the present invention to produce a free layer, suitable for both CPP configuration GMR sensors and TMR sensors that will enhance their respective MR ratios, while maintaining good magnetic softness (low coercivity) and providing an adjustable magnetostriction.